In lithography for the fabrication of semiconductors, the formation of finer patterns is required in step with increases in the degree of integration. For further reductions in pattern size, wavelength shortening of exposure light sources is indispensable. At present, lithography by krypton fluoride (KrF) excimer laser (wavelength: 248 nm) is becoming the mainstream, and lithography of 100 nm or finer feature size by argon fluoride (ArF) excimer laser light (wavelength: 193 nm) is also being put to practical use. Moreover, various short-wavelength radiation lithography techniques using fluorine dimer (F2) excimer laser light (wavelength: 157 nm), extreme ultraviolet (EUV), X-rays, electron beams or the like are also under development.
In such semiconductor lithography, a variety of coating films are used as resist films, each of which serves to form a resist pattern on a substrate by using the property that its solubility to an alkaline developer changes only at exposed areas under the action of an acid generated by light, and also as upper layers or under layers for such resist films. Example of the under layers include anti-reflective coatings for minimizing reflected light from substrates to precisely form resist patterns; planarizing films used as under layers for resist films so that, upon forming additional resist patterns over substrates on which patterns have been formed, the surfaces of the patterned substrates are planarized; and under layers in multilayer resists for transferring resist patterns by dry etching. These coating films are each formed by dissolving a copolymer for lithography, said copolymer being equipped with the function of the corresponding coating film, and other additives in an organic solvent to prepare a coating fluid, and applying the coating fluid to a substrate by a method such as spin coating, and then eliminating the solvent optionally with heat or the like. In addition to optical properties, chemical properties and physical properties, such as coating properties and adherence to under layers, all required for resist films and anti-reflective coatings, these copolymers for lithography are also required to be equipped with fundamental properties as copolymers for coating films such that they are free of particles detrimental to the formation of fine patterns.
Resist polymers include negative-type resist polymers and positive-type resist polymers. A negative-type resist polymer is lowered in its solubility to an alkaline developer under the action of an acid, while a positive-type resist polymer is enhanced in its solubility to an alkaline developer under the action of an acid. A positive-type resist polymer is formed including, as essential components, recurring units contacting nonpolar substituent groups that are removed with an acid to form polar groups soluble to an alkaline developer and recurring units containing polar groups for enhancing the adherence to semiconductor substrates, and if necessary, may also include recurring units that contain polar or nonpolar substituent groups to adjust the solubility to a resist solvent or the alkaline developer. As the recurring units containing polar groups for enhancing the adherence to substrates, hydroxystyrene units or the like are primarily used when a KrF excimer laser is used as an exposure source. When an ArF excimer laser is used, hydroxystyrene type units or the like absorb light of 193 nm wavelength so that (meth)acrylate units containing polar groups are now under investigation.
As known specific examples of such positive-type resist polymers, those useful with the KrF system include copolymers making combined use of acrylic acid monomers and styrene monomers (see, for example, JP-A-59-045439, JP-A-5-113667, JP-A-7-209868 and JP-A-11-065120) and polymers with some of hydroxystyrene units being protected with acetal (see, for example, JP-A-62-115440, JP-A-4-219757, JP-A-3-223860 and JP-A-4-104251), and those useful with the ArF system include copolymers of (meth)acrylic acid monomers having the lactone structure (see, for example, JP-A-9-073173 and JP-A-10-239846).
Known as polymers for anti-reflective coatings are polymers obtained by copolymerizing aromatic-nucleus-containing vinyl compounds such as styrene, styrene derivatives and anthracenylmethyl(meth)acrylate with acrylamide derivatives or hydroxyl- or epoxy-containing vinyl compounds and if necessary with alkyl(meth)acrylates or the like (see, for example, JP-A-2000-313779, JP-A-2001-027810, JP-A-2001-192411 and JP-A-2001-226324). As polymers for planarizing films, are known copolymers of hydroxystyrene with polymerizable compounds such as styrene, alkyl(meth)acrylates, hydroxyalkyl(meth)acrylates and the like (see, for example, JP-A-2003-057828).
If unreacted monomers, polymerization initiators, chain transfer agents, their coupling products or the like, or impurities added or formed upon polymerization reactions remain in copolymers for semiconductor lithography such as these resist polymers and polymers for anti-reflective coatings, they may evaporate and damage aligners in lithography, or during storage as copolymers or in the form of compositions for lithography, they may undergo polymerization to form substances that may cause pattern defects. A purification step is, therefore, needed to eliminate these impurities upon production of the copolymers.
As a purification method for such copolymers, there is known a method in which a polymerization mixture and a poor solvent are mixed to precipitate a solid (see JP-A-2001-109153). This method is, however, accompanied by a problem that impurities such as unreacted monomers cannot be fully eliminated when precipitation is conducted only once. Precipitation may be conducted twice or more, but in such a case, it is necessary to repeatedly perform operations such as precipitation, filtration and dissolution.
As simpler methods, there are hence known methods in which a solid obtained by precipitation is washed and filtered by dispersing it in a poor solvent or a mixed solvent of a poor solvent and a good solvent. Disclosed are, for example, methods in which a solid obtained by precipitation is dispersed in a poor solvent or a mixed solvent of a poor solvent and a good solvent, heated, and then collected by filtration (see JP-A-2002-201210 and JP-A-2002-229220) and a method in which a solid obtained by precipitation is dispersed in a poor solvent, the solvent is removed from the resultant dispersion by a centrifugal separator, a small amount of the poor solvent is added, and the solid is rinsed in the centrifugal separator (see JP-A-2003-231721). The methods in which a solid is dispersed in a solvent and then heated are, however, accompanied by a problem that a portion of the resulting copolymer is dissolved into the solvent and the yield of the copolymer is lowered.
Further, precipitation or washing is generally conducted using a vessel equipped with an agitator that has only a mixing function. Filtration, on the other hand, is conducted using a filter tank having a filter medium, a filter press, a centrifugal filtration apparatus, or the like. Whichever method is employed, it is accordingly necessary to transfer a slurry from a precipitation tank or washing tank to a filter tank after conducting precipitation or washing. When precipitation and washing are conducted repeatedly, it is necessary to once take a filter cake out of a filter tank subsequent to its formation in the filter tank, to dissolve the filter cake, and then to drop or otherwise return the resulting solution into a precipitation or washing tank. These operations involve problems that they are irksome, require many equipment, are inefficient, and further, are vulnerable to contamination from the outside.
In addition, the solid obtained by precipitation and containing the resultant copolymer is generally in the form of fine particles. With a conventional filter apparatus, the resulting filter cake and colloidal polymer deposit on the filter surface so that the filtration rate is reduced, leading to a problem that a long time is needed for each filtering operation. If only the precipitation step is conducted to complete the work in a short time, another problem arises in that impurities cannot be removed fully.
Moreover, the temperature of a slurry tends to fluctuate upon precipitation and washing due to the heat of dissolution into the solvent. During filtration, especially when filtration is conducted under reduced pressure, the temperature of the resulting filter cake tends to drop promptly. According to the conventional methods, however, it is difficult to control the temperature of a filter cake upon precipitation or washing. If the temperature varies in the purification step as described above, the removal rate of low molecular weight substances which are caused to dissolve together with impurities into a precipitation solvent or washing solvent varies. In purification of industrial scale, the properties of the copolymer, such as its weight average molecular weight and molecular weight dispersion, vary lot-to-lot, thereby making it difficult to produce the copolymer with uniform properties and good reproducibility. This causes a further problem that its performance in lithography becomes unstable. These slight differences in properties lead to variations in the characteristics of lithography. With the advance of micro lithography, a desire has therefore arisen for the development of a process for the production of a copolymer having stable quality with little lot-to-lot variations.